gpg: Avoid publishing the GnuPG version by default

* g10/gpg.c (main): initialize opt.emit_version to 0
* doc/gpg.texi: document different default for --emit-version

The version of GnuPG in use is not particularly helpful. It is not
cryptographically verifiable, and it doesn't distinguish between
significant version differences like 2.0.x and 2.1.x.

Additionally, it leaks metadata that can be used to distinguish users
from one another, and can potentially be used to target specific
attacks if there are known behaviors that differ between major
versions.

It's probably better to take the more parsimonious approach to
metadata production by default.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>